As the lumbar spine ages, disc degeneration occurs. This degeneration causes a reduction in the vertical height of the disc, and a diminution of its viscoelastic properties. The profile of the spine also changes with age. The swayback curvature of youth becomes the flat-back of old age. This results in increased biomechanical stress on the posterior side of the spine.
With a recent increased understanding in the biomechanics of the spine, it is acknowledged that maintenance of the normal curvature of the lumbar spine is preferable. When spinal fusions are considered, it is important to re-establish the normal biomechanical arrangement, and to restore the sagittal profile of the spine to obtain optimal results. Arthritic changes in the facet joints following disc degeneration can cause mechanical back pain. If they become excessive, these arthritic changes can cause spinal stenosis.
For these reasons, several prior art techniques have been used that remove the degenerated disc, distract the disc space, and rigidly bond the upper and lower adjacent vertebrae together. Initially, this was accomplished by inserting pieces of bone having cortex and marrow cut from solid bone locations, such as the wing of the pelvis or the fibula. Such grafts were only able to support around 430 lbs. of force within the disc space, however, and compression forces up to 1,850 lbs. can be experienced by a human when, for example, bending over to pick up a 25 lb. child. After experiencing such high compression forces, these grafts tended to collapse and lose their ability to fix and stabilize the spinal motion segment by distracting the disc space.
To deal with this problem, metal cages packed with bone or ortho-biological compounds (osteoinductive/osteoconductive) capable of fusing with the adjacent vertebrae were inserted in the distracted disc space. Such cages were able to withstand the larger compression forces while allowing the bone inside of the cages to fuse with the adjacent vertebrae. The cages were typically constructed of titanium mesh cylinders, screw-in bullet-like metal cages with external threads, or rectangular cages made of carbon fiber.
These cages were typically inserted into the disc space after the space had been distracted and/or drilled by a separate tool to form a niche for the cage and to broach through the cartilage and into the boney tissue to promote fusion. For purposes of this disclosure “broaching” refers to cutting through the cartilage of adjacent vertebral endplates and into the boney tissue of the vertebrae. The process of separately broaching and distracting the disc space, however, requires multiple steps of inserting and removing various drills, broaches and/or distracters into the disc space, causing direct or indirect damage to the load-bearing endplates of the adjacent vertebrae, weakening them and jeopardizing the fixation of the interbody fusion construction.
Because of the openings in the cages, another problem with the prior art cages were that small pieces of bone or ortho-biological material were capable of spilling out of the cage and into the soft tissue surrounding the surgical opening before the cage was placed between the vertebral endplates. In this case, heterotrophic ossification, or bone growth, could occur in the access port of the surgical wound or possibly near the exiting segmental nerve, resulting in bony nerve entrapment and tremendous pain and complications.
Other prior art methods of distracting the disc space included inserting a semi-rigid, U-shaped object with internal teeth. A round object with a larger diameter than the interior space of the object was then inserted into the interior space between facing interior teeth. By moving the round object further into the interior of the object, the legs of the U-shaped object were pushed apart, thereby distracting the disc space. These U-shaped devices, however, typically broke due to the forces exerted by the round object and the adjacent vertebrae.
In addition to the above problems with inserting the prior art cages, the cages are susceptible to movement within the disc space once they are inserted. This movement can damage the biological growth of the fusion, due to shear forces on the vascular ingrowth nourishing the endosteal bone growth—limiting the development of fusion of the bone inside of the cage which is necessary to stabilize the adjacent vertebrae, resulting in looseness and bone graft collapse. To this effect, several prior art cages were constructed with short spikes or points to stabilize them within the disc space, but which were not long enough to broach through the cartilage into the boney tissue of the vertebrae.
Moreover, if the compression forces are not withstood and the disc space is not held in a distracted state by a strong cage, the cage may collapse, leading to looseness, instability and further failure of fusion due to movement. However, if the cage is overly rigid and strong, such as a threaded cage, it may shield the bone inside of the cage from the normal stresses and strains that bone needs to develop into weight bearing, trabecular bone, which will fuse with the adjacent vertebrae in a strong and rigid fashion. This failure to satisfactorily promote fusion may also lead to looseness and instability of the cage or fusion construct.
Therefore, a need exists to provide an interbody device or a method of inserting such a device that solves one or more of the problems described above.